Since the rotational speeds of a camshaft and a crankshaft differ by a factor of exactly two, an angular relationship between the camshaft and the crankshaft may be defined with each revolution of the camshaft and with every second revolution of the crankshaft. This may occur, for example, by allocating a reference mark on the crankshaft to a reference flank on the camshaft. This angular relationship is also referred to as the camshaft position or the angular position of the camshaft.
A camshaft adjustment allows a mechanical variation on the valve trigger times at the intake/exhaust channels of a cylinder chamber, i.e., a variation on the angular position of the camshaft. The required adjustment of the camshaft in the direction of early or late is accomplished via a vane adjuster or the like, for example, via oil pressure. The flow of oil to the individual chambers is controlled by solenoid valves, which are addressed via a pulse-width-modulated signal, starting from the control unit. The camshaft must reach a setpoint value with sufficient accuracy within a period of time which is to be maintained. If this is not the case, then it is a misadjustment. Misadjustments during the variable camshaft adjustment have a negative effect on the exhaust gas, among other things, due to suboptimal combustion or even result in engine misfire. In addition, the driver will find fault with a lack of torque or a lack of power and loss in engine smoothness, for example, jerking, which occurs due to faulty camshaft positions and the resulting differences in filling between the banks of cylinders.
Camshaft control may be diagnosed by checking the adjustment angle of the camshaft. For this purpose, the difference between the setpoint value and the actual value is compared with applicable threshold values. If the applicable period of time exceeds or falls below the threshold values, then an error may be set in the camshaft adjustment unit. It is possible for a camshaft adjustment error to be set whenever the actual angle position is outside of an allowed tolerance band after a decay time has elapsed. Using such a camshaft diagnosis, a general adjustment error in the camshaft may be diagnosed.
During a cold start of the internal combustion engine, it is advantageous to adjust the camshaft in such a way that particularly hot exhaust gases result to heat up a catalytic converter in the exhaust gas system as rapidly as possible. Such an operating mode is also referred to as CSERS operating mode (“cold start emission reduction strategy”). The angular position of the camshaft during this CSERS operating mode thus differs from the angular position the camshaft would assume if the CSERS operating mode were not active, i.e., at a warm start of the internal combustion engine.
A draft bill of CARB, a U.S. regulatory authority [California Air Resources Board], requires a differentiation of the diagnostic malfunction of the camshaft adjustment. It is necessary in particular to diagnose whether the angular position of the camshaft during a CSERS operating mode differs from the angular position the camshaft would assume at an operating point which is otherwise the same if the CSERS operating mode were not active if the setpoint value of the angular position in an active CSERS operating mode would be different, i.e., in an inactive CSERS operating mode.